add round-robin arbiter

heavycomps/heavycomps/roundrobin.py

@@ -0,0 +1,24 @@
+from nmigen import *
+
+
+class RoundRobin:
+    def __init__(self, n):
+        self.n = n
+        self.request = Signal(n)
+        self.grant = Signal(max=n)
+
+    def elaborate(self, platform):
+        m = Module()
+        n = self.n
+        if n > 1:
+            with m.Switch(self.grant):
+                for i in range(n):
+                    with m.Case(i):
+                        with m.If(~self.request[i]):
+                            for j in reversed(range(i+1, i+n)):
+                                t = j % n
+                                with m.If(self.request[t]):
+                                    m.d.sync += self.grant.eq(t)
+        else:
+            m.d.comb += self.grant.eq(0)
+        return m